In the field of data transmission by optical waveguides, a distinction is usually made between two main applications:
long-distance optical transmission for telecommunications; and
short-distance optical transmission for data transmission on board aircraft and ships or in industrial installations.
The first application relates to optical fibre cables in which each optical conductor or fibre conveys data different from that of the other fibres. Connections must be made in the fibre-to-fibre configuration and a multi-contact connector must provide individual one to one connection of the fibres.
The second application relates to optical fibre cables in which each conductor is constituted by a plurality of fibres. Within any one conductor, each fibre conveys the same data as neighbouring fibres. Connection is made on an overall conductor-to-conductor basis via an optical mixer which is usually constituted by a glass cylinder with two optical indices, the cross-section of the core of said cylinder covering the hexagonal bundle constituted by the fibres of the cable. The lower the packing fraction, the higher the connection loss which results therefrom. Said fraction is the ratio between the sum of the areas of the cores of the fibres and the area of the cross-section of the core of the mixer. Said loss is generally about 50%, i.e. 3 dB and is added to that which results from the mechanical production and connection tolerances of the connector parts. Therefore, in this case too, fibre-to-fibre connection is preferable.
The present invention therefore relates to both the main applications mentioned hereinabove.
When forming fibre-to-fibre connections, the difficulty arises from the need to align the optical fibres sufficiently accurately if coupling losses are to be reduced as far as possible, e.g. losses of less than 1 dB. If the fibres have, for example, a core diameter of 100 microns surrounded by lower index cladding, with a difference in index lying between 5. 10.sup.-3 and 10.sup.-1, these devices must then allow the fibres to be positioned in the following conditions:
The distance between the adjacent ends of the fibres must be less than 20 microns; relative eccentricity must be less than 4 microns; relative inclination must be less than 1 degree. In the event that the index of the medium which separates the fibres matches, these tolerances become larger: 100 microns for distance and 10 microns for excentricity, inclination being unchanged. Due to the small diameter of the fibres, it is difficult to satisfy such tolerances.
That is why it may be advantageous to hold each fibre in one of the holding grooves formed by the adjacent lateral surfaces of cylindrical holding pins which are pressed against one another and form a plane layer which is pressed down onto a plane base surface. Such holding pins may be made of hard steel and be machined to great accuracy at a low cost. They are held parallel to one another and in contact in pairs by their lateral surfaces by suitable clamping means. The fibres are placed in suitable positions by guide means which guide the holding assembly constituted by the layer of holding pins.
The fibres may then be guided accurately since a very much stronger and more rigid assembly is guided and the transversal dimensions thereof are much greater than those of the fibres. These guide means may themselves be constituted by cylindrical pins made of a hard and rigid substance and which press laterally on either side of the holding assembly, and down onto the base surface. Another disposition which also uses holding and guide pins is described elsewhere, in the application for the first French certificate of addition No. 2 316 611, for "A fibre positioning device" equivalent to U.S. Pat. No. 4 050 783 (inventor: Andre Tardy). In accordance with said other disposition, the holding pins are spaced out all around an axis, the holding assembly being surrounded by guide pins, the whole being surrounded by clamping means.
Hereinafter, the assembly formed by two complementary "connector parts" and fitted together connected to the end of a cable to connect two cables together is called a "connector". The set of components of a connector part which holds the end of a single fibre is here called a "contact part". Therefore, each connector part has as many contact parts as there are fibres in the cable. These contact parts are disposed in a "connection component" at the front of the connector part, and the rear of the connector part includes a joining component which allows it to accommodate and fix the optical fibre cable which contains the largest portion of the length of the fibres. The present invention applies more particularly to the case in which the contact parts are formed by the front ends of the holding grooves for receiving the fibres to be connected.
Known fibre-to-fibre connectors often have disadvantages which are apparent when it is required to connect a cable to a connector part. Indeed, it is then a difficult and long job to insert the set of front ends of the fibres in the set of rear ends of the holding grooves. This is due to the fact that the fibres are generally disposed in the optical fibre cable for example in a hexagonal bundle in a round cable; this is different from the disposition of the holding grooves in the connection component, said disposition being, for example, a plane layer of equidistant parallel grooves and being chosen to facilitate forming and using said connection component. Obviously, this connection difficulty increases with the number of fibres of the optical fibre cable which can be connected by the connector part. It can prevent choosing as high a number of fibres as would be desirable.
The present invention aims to produce the connector part of a fibre-to-fibre connector including a component for joining it to an optical fibre cable in which said joining component facilitates connection of the connector part to a multifibre optical fibre cable and consequently makes it possible, where necessary, to increase the number of contact parts of the connector part.
In particular, it provides a connector part as defined hereinbelow.